It has been proved that a filtration with a reverse osmosis membrane which has been applied to various fields of water treatment including seawater desalination is superior to the filtrations with other membranes in terms of separation performance.
Referring to FIG. 1 and FIG. 2, a filtration with a reverse osmosis membrane is described below.
FIG. 1 and FIG. 2 are block diagrams respectively showing a filtration system using a reverse osmosis membrane.
As shown in FIG. 1, a preliminary filtration with a pressurized-type filtration membrane module 10 for MF (microfiltration) or UF (ultrafiltration) is generally performed before a filtration with a reverse osmosis membrane is performed. That is, the feed water (or pre-treated water produced through sand filtration) pressurized by the first pump P1 is filtered with the pressurized-type filtration membrane module 10.
The initial filtrate produced through the preliminary filtration is stored in the water tank 20.
Then, the initial filtrate stored in the water tank 20 is forwarded to the second pump P2 by the pressurizing pump P3, pressurized with a pressure higher than the osmotic pressure by the second pump P2, and then filtered by the reverse osmosis membrane module 30. The ions and molecules in the initial filtrate cannot pass through the reverse osmosis membrane, and only pure water passes through the reverse osmosis membrane.
Since the feed water (or pre-treated water) such as seawater is filtered by the reverse osmosis membrane module 30 after the solids therein is removed by the pressurized-type filtration membrane module 10, the reverse osmosis membrane in the reverse osmosis membrane module 30 can be prevented from being damaged due to the solids.
However, a considerable amount of energy is required for the second pump P2 to pressurize the initial filtrate from the water tank 20 with the pressure higher than the osmotic pressure. For this reason, the filtration system illustrated in FIG. 2 was suggested to reduce the energy consumption.
According to the filtration system illustrated in FIG. 2, there is no water tank 20 between the pressurized-type filtration membrane module 10 and reverse osmosis membrane module 30. Instead, the feed water (or pre-treated water) to be introduced into the pressurized-type filtration membrane module 10 is pressurized with much higher pressure so that the initial filtrate produced by the pressurized-type filtration membrane module 10 can be discharged from the pressurized-type filtration membrane module 10 in a pressurized state. Since the initial filtrate produced by the pressurized-type filtration membrane module 10 is in a pressurized state in some degree, relatively small energy is required to pressurize the initial filtrate with a pressure higher than the osmotic pressure if the initial filtrate is directly pressurized by the second pump P2 without being stored in the water tank 20. This is called “direct feed” or “tankless feed.”
However, the filtration system illustrated in FIG. 2 has the following drawbacks.
First, since the “direct feed” filtration system illustrated in FIG. 2 requires the feed water (or pre-treated water) to be pressurized with higher pressure than the filtration system illustrated in FIG. 1, the risk of damage of the filtration membrane in the pressurized-type filtration membrane module 10 increases.
Second, since the filtration membrane of the pressurized-type filtration membrane module 10 is contaminated as the preliminary filtration is performed, it is necessary to clean the filtration membrane of the pressurized-type filtration membrane module 10 by performing a backwash periodically. The reverse osmosis membrane module 30 has to be stopped during the backwash process, which is detrimental to the filtration efficiency. Furthermore, after the backwash process, lots of energy is consumed to restart the first and second pumps P1 and P2 which were shut down during the backwash process, because lots of energy is required to restart a stopped pump.